1. Field of the Invention
The present invention relates generally to a process for treating AB.sub.5 nickel-metal hydride battery scrap and, more particularly, to a process for recovering purified positive and negative electrode components from such battery scrap for recycling within the battery industry or reuse in a variety of other end-applications.
2. Description of the Prior Art
The world's first rechargeable battery was the lead-acid cell invented by Gaston Plante in 1860. At roughly the turn of the century, alkaline accumulators such as nickel-cadmium (NiCd) and nickel-iron replaced the lead-acid battery as the most commonly used type of rechargeable battery. Although NiCd rechargeable batteries have achieved exceptionally wide acceptance since the 1950's, alternative battery electrode materials are presently urgently being investigated and implemented for a variety of reasons.
First of all, NiCd batteries pose a serious environmental threat due to the problems associated with the disposal of Cd. It is estimated that more than half of the 1,775 tons of Cd in the U.S. municipal waste stream in 1987 came from NiCd batteries which do not pass the EPA's materials toxicity test. In addition, when NiCd batteries are burned with other waste, the resulting ash is so contaminated with Cd that it must be classified hazardous. Although spent batteries represent less than 1% of household waste content, their contribution to the metal feed of a typical incinerator can be higher than 30%. The hazardous material designation means extra work and expense for government and industry and also minimizes available disposal and recycle options for municipalities. Over 30 states have now enacted battery recycling legislation with the intent to limit Cd disposal.
Another reason why NiCd replacement alloys are being actively pursued has to do with the improved electrochemical properties of batteries of alternative design. In nickel-metal hydride (Ni--MH) batteries, for example, a solid-state reaction mechanism is utilized which permits a simple and compact construction resulting in high power density caused by rapid mobility of the hydrogen atom, and effective protection against overcharging and overdischarging. In addition, Ni--MH batteries have no memory effect. NiCd batteries, if not discharged completely before being recharged, can "remember" the partial charge level as the full charge point and often fail to return to full power on the next recharge.
Research on metal hydride alloys began in earnest during the 1970's triggered by the potential use of these materials in heat pumps and as energy carriers. It was found that intermetallic compounds such as LaNi.sub.5 and TiFe can absorb large amounts of hydrogen gas and desorb it under ambient pressure at room temperature. Water from an aqueous electrolyte can be reduced electrochemically using these materials while the hydrogen evolved can be stored as a hydride and reoxidized to form water. Consequently, the formation of hydrogen is not an unwanted side reaction but part of the energy storage system.
A great deal of investigation has been carried out on LaNi.sub.5 alloys which are commonly referred to by the generic name AB.sub.5 due to their stoichiometry. LaNi.sub.5 is converted to LaNi.sub.5 H.sub.6 during hydriding. In battery manufacture, LaNi.sub.5 is used as the active material of the negative electrode and is converted into LaNi.sub.5 H.sub.6 during the charge. LaNi.sub.5 powder has high catalytic activity, a high sorption rate for hydrogen in the lattice, and exhibits favorable electrical conductivity. All in all, LaNi.sub.5 has shown to be a promising electrode material for a rechargeable battery except that the storage capacity of LaNi.sub.5 can decrease drastically after repeated charging and discharging. Decay in capacity is caused by degradation of the electrode due to the formation of La(OH).sub.3 and can be reduced to acceptable levels by encapsulation of the alloy or by the addition of Co, Si, Al and Zr.
Because of the electrochemical and environmental advantages of Ni--MH batteries over NiCd batteries, it is highly likely that Ni--MH batteries of the AB.sub.5 variety will achieve widespread use in consumer and automotive applications. If this is so, recycle technology will be extremely important to both government and industry if the Ni--MH battery is to in fact achieve the extent of use presently envisioned. However, to date no technology is available for effecting the recycling of AB.sub.5 Ni--MH batteries.
Consequently, a need exists for a commercially and technologically viable process for treating AB.sub.5 Ni--MH batteries to recover purified positive and negative electrode components of the batteries for reuse either within the battery industry or in other industrial end-applications.